1. Field of Invention
The present invention relates generally to surgical implants, and more specifically to a bone screw with features that permit bone ingrowth.
2. Background Information
Fasteners are used in a variety of surgical applications including affixing implants to tissue and affixing tissue to tissue. A common surgical fastener is a bone screw, Which comprises threads designed to achieve mechanical purchase within bone. Bone screws exist in a variety of styles including pedicle screws, fracture fixation screws, and screws for securing bone plates. Bone screws are widely used because they are easy to insert, easy to produce, and under certain conditions provide adequate mechanical purchase. However, in applications involving highly porous, osteopenic or osteoporotic bone the efficacy of bone screws is substantially reduced. Design optimization of bone screws for these applications is challenged by competing constraints; the need for larger (i.e. coarser) threads versus the need for adequate bending strength (e.g., provided by the minor diameter of a screw) versus the maximally allowable major diameter (often limited by anatomy or mating component geometry). Further, bone screws function in a dynamic biological environment in which the supporting bone and tissue undergoes constant remodeling which can adversely affect the stability of the bone screw through time.
Design features aimed at improving the efficacy of bone screws without affecting the conventional thread form have focused on enhancing the implant-bone interface. Proposed enhancement have included use of biologically favorable metals (e.g. titanium), application of favorable coatings (e.g. hydroxyapatite), and surface texturing.
Other design solutions departing from conventional thread forms have incorporated features including expanding tips, trabecular metal, etc. These solutions may compromise the mechanical integrity of the screw.
Despite these proposed solutions, screw failure (e.g. loosening, breaking) is still a frequent clinical occurrence. Thus, the need exists for a bone fixation device that can be delivered as easily as a conventional bone screw, allows for bone ingrowth, and maintains adequate mechanical performance (e.g., pullout, bending).